The standard I2P installation includes about 20 versions of the library for different platforms,
each about 50KB, inside the jbigi.jar file.
The initialization of the JBigI library, including CPU identification, selection, and extraction
of the correct loadable module, is handled by the
NativeBigInteger class.
If no module is available for the current platform, the standard
Java Math library's BigInteger modPow()
is used.

Rebuilding and Testing JBigI

Following are the instructions to build a new jbigi library for your own platform
and testing its performance.

Requirements

This works on Linux, and with a few changes in build.sh probably also on
other platforms. FreeBSD has also been reported to work too. On Kaffee the
speedup is very small, because it already uses native BitInteger internally.
Blackdown seems to cause strange errors. Because you are going to do
compilation, you need JDK; JRE won't work.

The required code is available in monotone database and the latest source tarball.

The GNU MP Bignum library (libgmp) needs to be installed, if it isn't
included in your OS / distribution or installed already, it can be received from
http://gmplib.org/#DOWNLOAD. Even if you
have already installed it as binary, it might still be worth a try to compile
GMP yourself, since then it will be able to use the specific instructions of
your processor. The latest GMP may also
be used instead of GMP 5.0.2, but it hasn't been tested by us.

Step-by-step instructions

Look at your running environment on the logs.jsp page.
There should be one of two status messages for JBigI - either
Locally optimized native BigInteger loaded from the library path
or
Native BigInteger library jbigi not loaded - using pure java.
If the native BitInteger library was NOT loaded, you definitely need to
compile your own.
Certain platforms, such as OS X, OpenSolaris, and 64-bit systems,
may require you to compile your own library.
If the BigInteger library was loaded, do at least the next step to see
what your performance is.

Look on http://localhost:7657/stats.jsp
to see what the lifetime average values for crypto.elGamal.decrypt and
crypto.elGamal.encrypt are. The numbers are times in milliseconds. Copy these somewhere so you can compare
them later on.
The network average for encrypt time is about 20ms.
If your encrypt time is less than 50ms for a relatively new processor, or less than 100ms
for an older processor, and the native BigInteger library was loaded, you are probably fine.

Get the latest released source code of I2P from
the download page, or get the cutting-edge source
out of the monotone database mtn.i2p2.de

Inside the source tree change directory to: core/c/jbigi

Read the README file.
If you have a /usr/lib/libgmp.so file, you do not have to download GMP.
Use the 'dynamic' argument to build.sh.
Otherwise, you must download GMP version 5.0.2 from
from http://gmplib.org/#DOWNLOAD, saving it to gmp-5.0.2.tar.bz2.
If you decide to use a newer version, change the VER= line in core/c/jbigi/build.sh.

Take a look at build.sh, if your JAVA_HOME
environment variable is set and you are using Linux then it might just work.
Otherwise change the settings. Remember, you need the Java SDK installed.

Run build.sh (if you downloaded GMP) or
build.sh dynamic (if you have /usr/lib/libgmp.so).
Maybe the build spewed out some errors of missing jni.h and jni_md.h files.
Either copy these files from your java install into the core/c/jbigi/jbigi/include/ directory,
or fix $JAVA_HOME.
You can run the build.sh from the core/c/ directory which will
build all available jbigi libs into a jbigi.jar.
A file named libjbigi.so should be created in the current
directory. If this doesn't happen and/or you get errors then please report
them.

Follow the instructions in core/c/README to install the library and run
the speed test.
Read the final lines of the speed test's output for some additional
info, it will be something like this: